The use of metallic flake pigments, such as aluminum flake pigments, in decorative coatings to give the coating a metallic effect is widespread. The metallic effect is particularly popular with customers in the automotive market where “glamour finishes” are desired.
Automotive coatings can utilize a single, uniformly pigmented layer. Alternatively, they can have two distinct layers, i.e., a first, highly pigmented layer (basecoat) and a subsequently applied coating layer with little or no pigmentation (clearcoat). The two-layer coating is known in the industry as “basecoat/clearcoat”. Basecoat/clearcoat coatings impart a high level of gloss and depth of color that can result in a particularly appealing look. Metallic flake pigments typically are incorporated into the basecoat composition.
Waterborne automotive paints are gaining widespread usage in the automotive industry due to concerns over organic solvent emissions during coating application and curing processes. However, waterborne paints have the disadvantage of using a medium that can be corrosive to metallic flake pigments. For example, hydrolysis of the metal pigments can occur in waterborne paints. Additionally, the pH of typical waterborne acrylic coating compositions can range from 8.0 to 9.0, and typical polyurethane coating compositions can have a pH typically ranging from 7.5 to 8.0. In a basic pH environment, the aluminum pigment can be oxidized. The oxidation is a form of corrosion that destroys the metallic pigmentation properties of the mirror-like particles. When a paint with oxidized metallic flake pigments is coated onto a substrate, the coating shows discoloration and diminished metallic effect.
Additionally, the hydrolysis or oxidation of the metallic surfaces in waterborne paints results in the evolution of hydrogen gas. The amount of hydrogen gas evolved is indicative of the amount of oxidation (i.e., corrosion) of the metallic pigment. The hydrogen gas can accumulate under pressure if the coating composition is stored in a closed container.
Hydrolysis of aluminum pigment in the presence of water can accelerate over time due to continuous contact with the basic pH environment of the coating composition. Coating compositions containing metal flake pigment are often stored for 6 months or more before application, which can result in significant corrosion of the pigment. If this corrosion remains unchecked the coating composition can be rendered unusable.
Considerable work has been done in the industry to treat or “passivate” metal pigment surfaces to prevent corrosion of the metal surface by water in waterborne coating compositions. For example, it is known to apply a chrome coating over aluminum pigment surfaces to prevent the corrosion and hydrogen generation described above. However, chrome can be toxic and, therefore, special handling and disposal procedures are required for such chrome coated metallic pigment particles.
Therefore, it would be advantageous to provide a coating composition that could be used to passivate metal pigments while reducing or eliminating altogether at least some of the problems associated with known passivation procedures.